On the surface, there are many similarities between the ZF 8HP transmission family and the GM 8L90. Namely both use 4 gear sets, 5 shifting elements (3 clutches and two brakes), off axis pumps, and have roughly the same gear ratio spread at about 7:1 overall spread. The saturation dive is not about dealing with things that are on the surface. To be entirely honest as the details started to emerge on the GM 8L family of transmissions I suspected that it would end up being a ZF licensed design, the ZF 8HP after all is a very good design in my opinion. But the abstract of the paper that we managed to snag before SAE took it down revealed one very important detail – all 3 of the clutches were located in front of the planetary gear sets very much unlike the ZF design.

The amount of information available publicly on the 8L90 is very sparse and is right now limited to the the SAE papers the engineers responsible for the transmission wrote and the patent granted to GM. The folks at SAE want money for the paper, but the patent #8105196 is available as a matter of public record from the USPTO. The structure of this particular saturation dive is therefore going to be a bit different from the past ones. Please refer to the earlier saturation dives for a rehash of the basics of gears. The drawing from the patent filing is shown below

GM 8L90 patent drawing

The stick diagram

Since the patent diagram is not the easiest thing in the world to read, the stick diagram for the transmission is shown below. The output shaft of the torque converter is the input shaft for this gear arrangement.

Stick diagram for the GM 8L90

As we see from the stick diagram, there are 4 simple planetary gear sets. The shift elements function as follows

Brake A when activated grounds the sun gears of gear sets 2 and 3

Brake B activation grounds the ring gear of gear set 3

Clutch C activation ties the input shaft to the sun gear of gear set 4

Clutch D activation ties the ring gear of gear set 1 to the sun gear of gear set 4

Clutch E activation ties the ring gear of gear set 2 and the syn gear of gear set 1 to the sun gear of gear set

Additionally, the following rigid links exist

The input shaft is rigidly connected the planetary carrier of gear set 2

The sun gear of gear set 1 and the ring gear of gear set 2 are connected together

The sun gears of gear sets 2 and 3 are connected together

The planetary carriers of gear sets 1 and 4 are tied together – which in turn is the output shaft of the transmission

The planetary carrier of gear set 3 is tied to the ring gear of gear set 4

Clearly while there are similarities, the planetary gear arrangements for the ZF 8HP and the GM 8L90 are actually quite distinct in layout. The stick diagram for the ZF 8HP is shown below.

Stick diagram for the ZF 8HP

The similarities between the 2 layouts are enumerated as follows

Rigid links between 2 sun gears, but between gear sets 2 and 3 for the GM design vs. gear sets 1 and 2 for the ZF design

Rigid link between planetary carrier of a gear set and the ring gear of gear set 4, but between gear sets 3 and 4 for the GM design vs. gear sets 1 and 4 for the ZF design

Rigid link between the input shaft and the planetary carrier of gear set 2

The major difference is that the ZF 8HP uses two rigid links between the sun gears and ring gears of adjacent gear sets, the 8L90 uses one such link and has a separate rigid link between the planetary carriers of gear sets 1 and 4. Therefore the GM 8L90 is a unique layout that is different from the ZF 8HP but there are similarities that are quite obvious as well.

First gear

The first gear is achieved by engaging both brakes A and B and clutch C. Engagement of brakes A and B locks the sun gear and the ring gear of gear set 3 to ground, which means that the planetary carrier of gear set 3 is tied to ground as well. Since this planetary carrier is rigidly tied to the ring gear of gear set 4, this means that the ring gear of gear set 4 is stationary. The engagement of clutch C ties the input shaft to the sun gear of gear set 4. This sets up an underdrive gear with a ratio of

(1)

1st =

S4+R4

S4

= 4.5517

We know that the gear ratio is around 4.55, and also that given the torque capacity requirements GM is more than likely using 4 pinions for higher torque rating similar to the ZF design, therefore (S4+R4) has to be divisible by 4. Also (R4-S4) has to be an even number. A candidate tooth count is S4=29 and R4=103, which leads to a feasible arrangement with 4 37 tooth planetary pinions and a first gear ratio of 4.5517, which is quite close to the known ratio. Since S4+R4 = 132 is divisible both by 4 and 3, for the lighter duty versions of the 8L family it is possible for GM to reduce the number of planetary pinions to 3 for applications that don’t quite require the 1000 Nm the 8L90 is capable of.

For the first gear operation, the GM 8L90 and ZF 8HP are therefore kinematic equivalents, but there is one very significant difference, the GM design places the brakes right next to the output gear set. The reason it matters is because at full torque (1000 Nm) and a 4.55 gear ratio, the 2 brakes are reacting a total of 3550 Nm of torque. The reaction torque in the ZF design has to pass through the outermost tube to the front of the transmission, i.e. approximately a 1.5 kg shaft assuming that the wall thickness is 3 mm, the length is approximately 350 mm and the inner diameter is around 180 mm. Now 1.5 kg does not sound like a lot, the problem is it is at a very large diameter, and rotational inertia is proportional to the square of the diameter. The rotating inertia of the ZF shaft is approximately 0.01 kg-m^2 while the GM design is around a tenth of that because it is at a much smaller diameter.

The input shaft in case of the GM design ends up being the outermost shaft, but the input shaft only carries engine torque while the outermost shaft in case of the ZF design has to carry 3.7 times the engine torque. Also, it appears that the length of the outer shaft in case of the GM design will be shorter than the ZF design.

The lower inertia of the GM design will shave a few precious milliseconds off the shift time, and in a world where the burgerkingring times are important, over a course of a lap the precious milliseconds can add up to a second. As the B&B we all know just how important the burgerkingring times are to market share. On a more serious note, this reduced inertia will also show up as a very small fraction of a mile per gallon for the EPA fuel economy.

Second gear

To shift up to second gear, both brakes A and B stay locked, clutch C is disengaged and E is engaged. Doing so connects the sun gear of gear set 4 to the ring gear of gear set 2. The sun gear of gear set 1 is grounded and the planetary carrier of gear set 2 is connected to the input. Therefore this sets up an overdrive cascaded with an underdrive. The ratio is

(2)

2nd =

(S4+R4)R2

S4(R2+S2)

= 2.9586

Knowing the gear ratio, and methods similar to the 1st gear estimation, a feasible and perhaps reasonable estimates for tooth counts is S2=42, R2=78 with an 18 tooth planetary pinion. Since S2+R2 = 120, it is possible to put in 3 or 4 pinions. Once again, the second gear operation is kinematically equivalent to the ZF 8HP. The earlier observations about overall transmission inertia being lower for the GM 8L90 still stand.

Third gear

The shift up to third gear is accomplished by releasing brake A, and engaging clutch E. Therefore shift elements B, C, and E are engaged. By doing so, the ring gear and the planetary carrier of gear set 2 turn at the same speed as the input, and therefore the sun gear of gear set 2 also turns at the input speed. Since this sun gear is linked to the sun gear of gear set 3, and the ring gear of gear set 3 is grounded by brake B, the carrier of gear set 3 is under driven with respect to the input. Therefore gear set 4 acts as a mixer module, with the sun gear rotating at the input speed, the ring gear turning slower than the input, forcing the carrier to turn at a speed that is slower than the input. The ratio is

(3)

3rd =

(S3+R3)(S4+R4)

S4R3+S3S4+S3R4

= 2.0745

Knowing the gear ratio, and methods similar to the 1st gear estimation, a feasible and perhaps reasonable estimates for tooth counts is S3=39, R3=77 with an 19 tooth planetary pinion. Since S3+R4 = 116, it is possible to put in 4 pinions. The operation of third gear is somewhat similar to the third gear of the ZF 8HP, this design uses gear sets 2, 3, and 4 while the ZF design uses gear sets 1, 2, and 4. Gear set 4 acts as a mixer module in both cases.

Fourth gear

The 4th gear up shift is achieved by releasing clutch C and engaging clutch D, i.e. shift elements B, D, and E are engaged. Engaging B and D at the same time means that the ring gear and sun gear of gear set 1 spin together with the sun gear of gear set 4. Since the planetary carriers of gear set 1 and 4 are linked together, all 3 members of gear sets 1 and 4 spin together at the output speed along with the ring gear of gear set 2 and the planetary carrier of gear set 3. Since the ring gear of gear set 3 is grounded, this causes the sun gears of gear sets 2 and 3 to be overdriven with respect to the output by a factor of approximately 3. This causes gear set 2 to act as a mixer module, with the planetary carrier as the input and sets up an underdrive gear. The ratio is

(4)

4th =1 +

S2R3

S3(R2+S2)

= 1.6910

The fourth gear power flow is significantly different from the ZF 8HP fourth gear power flow.

Fifth gear

Fifth gear is achieved by releasing clutch E and engaging clutch C, i.e. shift elements B, C, and D are engaged. The fifth gear power flow is quite challenging to understand, but I am going to give it the old college try. Since C and D are engaged, the following members turn at the input speed

The ring gear of gear set 1

The planetary carrier of gear set 2

The sun gear of gear set 4

The planetary carriers for gear sets 1 and 4 are rigidly linked together and turn at the output speed. Ring gear of gear set 3 is grounded because brake B is engaged. This sets up the following kinematic states

The sun gears of gear sets 2 and 3 are rotating at 2.16 times the input speed

Since the ring gear of gear set 3 is grounded, the planetary carrier of gear set 3 is therefore spinning at 0.73 times the input speed

Since the ring gear of gear set 4 is connected to the planetary carrier of gear set 3, gear set 4 becomes a mixer module with the sun gear spinning at input speed, the ring gear spinning at 0.73 times the input speed, and the planetary carrier being the output.

The 5th gear ratio is therefore

(5)

5th =

S1S3(R2+S2)(R4+S4) + S3R2(R4R1-S4S1) + R3S2S1(S4+R4)

S1S3(R2+S2)(R4+S4) + S3R2(R4R1x-S4S1) + R3S2S1S4

= 1.2682

Knowing the gear ratios, it is possible to back calculate a feasible gear parameters for gear set 1. After a little bit of work, S1 = 39, and R1 = 77 with 19 teeth planetary pinions. Therefore gear sets 1 and 3 appear to be identical in terms of number of gear teeth.

The kinematic state of gear set 4 is very much the same as the kinematic state for gear set 4 of the ZF 8HP, but the way the kinematic state is achieved is different. All 4 gear sets are used to achieve this ratio, albeit in a different manner than the ZF design.

Sixth gear

Sixth gear is achieved by releasing brake B and engaging clutch E, i.e. the 3 rotating clutches C, D, and E are all engaged but both brakes A and B are open. This means all members of all 4 gear sets turn at the same speed as the input. The ratio is therefore quite simply

(6)

6th =

1.0000

Seventh gear

Up shift to seventh gear is accomplished by releasing clutch E and engaging brake A. The engagement of brake A grounds the sun gear of gear set 2, while the engagement of clutches C and D connects the ring gear of gear set 1 to the input shaft. The ring gear of gear set 2 spins approximately 1.5 times faster than the input because the sun gear is grounded, the planetary carrier is the input, and the ring gear is the output. Therefore gear set 1 acts like a mixer module, with the sun gear rotating at approximately 1.5 times the input speed (due to the rigid connection to the ring gear of gear set 1), the ring gear turning at the input speed, and the planetary carrier being the output. The ratio is therefore decided by the ratios of gears sets 1 and 2 alone

(7)

7th =

R2(S1+R1)

R2(S1+R1)+S1S2

= 0.8467

Eight gear

Eight gear is achieved by disengaging clutch C and engaging clutch E, i.e. shift elements A, D, and E are engaged. Engaging clutches E and D at the same time causes all 3 members of gear set 1 to rotate at the same speed, and since the planetary carrier of gear set 1 is also the output shaft, this means that the ratio is decided strictly by the gear teeth count of gear set 2. The sun gear of gear set 2 is grounded, the planetary carrier is the input and the ring gear is the output. The ratio is therefore

(8)

8th =

R2

R2+S2

= 0.6500

Reverse gear

Reverse gear is achieved by locking both brakes A and B, and engaging clutch D. The sun gear of gears set 2 and the ring gear of gear set 4 are therefore grounded, the sun gear of gear set 1 spins at approximately 1.5 times the input speed (just as it does in 7th and 8th gears). The kinematic constraints imposed by the rigid link between the planetary carriers of gear sets 1 and 4, along with the actuation of clutch D which locks the ring gear of gear set 1 with the sun gear of gear set 4 causes the sun gear of gear set 4 to spin backwards at approximately 1.17 times the input speed, which means that the transmission output spins backwards but 3.908 times slower than the output

(Rev)

Reverse =

R2(S1S4-R1R4)

S1S4(S2+R2)

= -3.9080

What have we learned

Based on the available information, this article makes educated guesses at likely gear parameters for the GM 8L90. The likely gear parameters are

Gear set 1: Sun gear S1 = 39, Ring gear R1 = 77

Gear set 2: Sun gear S2 = 42, Ring gear R2 = 78

Gear set 3: Sun gear S3 = 39, Ring gear R3 = 77

Gear set 4: Sun gear S4 = 29, Ring gear R4 = 103

The gear ratio spacing is very good, the transmission feel of the GM 8L90 should be very competitive to the acclaimed ZF 8HP family of transmissions.

There are some obvious similarities between the gear arrangement of the GM 8L90 and the ZF 8HP but there are significant differences as well. These similarities and differences have been explained in this article. The one advantage of locating the clutches close to the hydraulic pump and the valve body is better shifting times since less fluid has to be moved in and out of the clutch pistons to apply and release the clutches. Also, this design is likely very competitive in terms of mass for a given torque capacity, and is better than the ZF 8HP design in terms of rotational inertia.

There are other advances made in this transmission design as well, especially with regards to the hydraulic pump design. This particular transmission features a “cylinder deactivation” of sorts for the pump, when line pressure demands are low (highway cruising) half of the pump can be shut down to achieve higher efficiency while still retaining the pump displacement required to deliver enough flow rate for fast shifts.

Conclusions

This is a very good design, hats off to the engineers at GM. The filing date on the patent is May 1, 2009 therefore props to the management at GM for letting this program move forward through the darkest days of their bankruptcy. This will be considered a seminal design in the history of automatic transmissions.

Having recently ordered a custom bicycle fitted with Rohloff’s 3 planetary 14 speed hub changer system I am enjoying these engineering breakdowns and explanations immensely. Thank you very much for taking the time to explain the inner workings in such detail!

I look forward to this becoming a separate part number, the better to match it to a crate LS9 for a plug-and-play supercar horsepower boost, although the engine’s warranty requirements are going to play hell with most engine bay installations.

I’ll be very curious about your experiences with it. That’s one of those pieces of kit I’ve love to design a bike around. However, finding a place that actually sells them is a prime case of “far and few between”.

And I’d love to tear one down, despite my practical knowledge being currently limited to Sturmey-Archer five-speeds.

On the wish list/build sheet, I also added one of Schlumpf’s Speed Drives for front shifter duties; just kick the crank-mounted button to switch the planetaries from direct drive to a 1.6 overdrive ratio. Sadly the recumbent’s frame is much longer than what Gates supplies its Carbondrive belts for, and I fear there isn’t enough support or interest in this particular community to raise enough money up front for them to make the molds necessary for a 2.7 meter belt plus extra for the necessary drive wheels.

I’m thinking about making a post on the forums about it when the new ride arrives; there are clearly a significant number of pedal-power fans among the B&B here.

Late last year I discovered a participating branch in the Inter-Library Loan (ILL) program which had a copy of the long out of print “Sturmey Archer Story” history book on its shelves. Sadly my request for a temporary loan was met with a $20 fee simply to send it out for a couple of weeks. I’d love to read that book, but at that price I’d like to have a hard copy of my own to keep around.

Kudos to the thorough technical analysis, but holy cow this is just another reason I’m glad I drive a manual transmission. 1 clutch, simple gear layout. When you need astronomy to describe the inside of your transmission, that’s probably a bad sign!

Now that we have had time to have real world experience concerning the 8L90 transmission and are now seeing many complaints from truck owners about this transmission. The mating of the 6.2 litre and the 8L90 seems to be a common thread with the complaints. Something is not right with this tranny.

> If you guys absolutely want a write up, I can do one – it will be even more convoluted than this one.

Just a tip if this happens. The intent in these is good, but frankly the calc/details detracts from the pedagogical experience.

For example, most readers are just interested in the main diffs between gear types, and a few insights into why one is used in a certain scenario vs another. Completeness is better for a reference/textbook than an interest piece.

To expand on this, I could read descriptions of how a transmission works (this connected to that), but what’s really useful is context for *why* certain design choices were made. Why is planetary used in all passenger autos instead of auto-sequentials? Do they rev-match to some degree and how? The kind of stuff the mechanically inclined sometimes wonder about but lack the inside background to trivially reason out.

To be absolutely honest I am still trying to get the hang of this whole writing thing. I do appreciate your feedback.

My intent with the saturation dive series of articles is to be reference/textbook type articles. These details are not available elsewhere on the internet and my intent is to provide them to the audience here. To that end, I work very hard to make these articles as factual as I possible and I tend to leave my perspective out of them to a large extent.

I will be doing a series of less detailed pieces on transmission design (for example why planetary gears) in the near future. So please be patient, give me a few more articles to find the right balance between detail and perspective.

Stick with the textbook style for these deep (saturation) dives and always show your work! They are fantastic and really flesh out a technical aspect to TTAC that has never really been strong before. I hope to better comprehend more of this article with a few rereads and supplemental research, but that’s better than the car blog fluff that typically gets circulated.

wow. if i had the time i could learn a ton from this article. as it is i clicked on it and read a bit and learned a bit too. fortunately my click counts as much as the click from a transmission geek. just love the detail on this site.

With an approximately 185 mm diameter, approximately 300 mm length, and a wild guess at a thickness of 3 mm, and assuming that the outer shaft of the ZF design is made out of aluminum, the weight is approximately 1.5 kg and inertia is approximately 0.01 kg-m^2.

For reference, a 2.0 L 4 cylinder DOHC engine comes it at 0.16 kg-m^2 or thereabouts.

And to answer your other questions – both ZF and GM probably have brute force automated methods (i.e. proprietary software) to crunch through thousands of these combinations and to narrow it down to a few dozen concepts for the engineers to evaluate.

Until everything else fails around the car. You mention Ultradrive as a jab at Chrysler, yet totally blow off ignition switches, intake gaskets, DexCool, and 5.3L Oil consumption problems along with piston slap on GM’s. Oh and least we forget the grenading transmissions on the LS4 powered cars.

Our ’94 325is received its 2nd GM automatic at 30,000 miles and its 3rd at 60,000 miles. We sold it with 92K. Those transmissions had ‘life-time’ fluid and weren’t set up for easy service. Turns out that 30,000 miles was the lifetime of the fluid, just like in any other transmission. The difference was the life span of the transmissions.

Does anyone know the plan of where they’ll implement this transmission? I’ve heard the Corvette & Camaro, and likely trucks. I assume it will find its way into Cadillacs. I don’t think I’ve heard anything about timetables, though.

What types of variations would it be feasible to make? Do they ever release lightweight or heavy-duty versions of transmissions for different applications? What type of limitations might there be?

V good article. I especially liked the logical deduction of number of teeth on the planetary gearsets. One suggestion to the author, when discussing gear ratios, it would be good if you could mark in red the powerflow from engine to output shaft. The power flow would be easy to visualize for the rapid reader.
Regarding 8l90 itself I think there are going to be times during shifts where there would be sudden loss in power at wheels. ex gear 1 to gear 2 shift. Imagine at low temps when fluid from C hasnt yet drained out and E is engaged, I think planetary gear set 2 would get locked and for a fraction of sec the driver would feel loss in power. Havent checked the other gear sets in detail, but just looking at the layout, am sure shift calibration is going to be really really tricky as number of planetary gear sets increase.

Great article but the GM 8L90 is not that much different from the ZF 8HP.
The powerflow in each gear of the GM 8L90 is the same as in the ZF 8HP.
GM took the ZF 8HP and swapped the position of the Gearsets 1 and 3.
They also changed the position of clutch D.
But both changes have no effect on the powerflow in each gear.
If you look at the big equation in this article for the GM 8L90 5th gear and change all the 1 into a 3
and all the 3 into a 1 you will have the same equation as for the 5th gear in your article about the ZF 8HP.
You can also do this for all the other gears with the same result.
Offcourse the clutches applied in each gear are for both transmissions also the same.

Perdo24, I always suspected that there was at least one reader who understood the equations :)

Of course there are similarities between the two designs, and yes the clutch actuation sequence is identical because I named the shift elements the same way for both the ZF 8HP and this design. But there is a little bit more in terms of planetary gear arrangement than a simple swap between gear set 1 and gear set 3, if you notice the connectivity of the planetary carriers between gear sets 1 and 4 is very different from the ZF 8HP. ZF 8HP uses a clutch D to selectively couple the two elements, while it is a hard connection in case of the GM design.

Having said that, yes the equations for the two designs are very similar.

To make a good comparison of the Powerflow in both transmissions
I redrawn the stick diagram for the GM 8L.
Using the same gearset numbers as you did.

Picture : http://oi59.tinypic.com/2iu5la9.jpg

As you can see in the picture only the position of Clutch D is different.
But when Clutch D is engaged the Powerflow in both transmissions is the same.
Clutch D is Engaged in gears 4 5 6 7 8 and reverse so the Powerflow in those
gears is for both transmissions the same.

In gears 1 2 3 Clutch D is Disengaged and this lets the planetary carrier (ZF 8HP)
or the ring gear (GM 8L) freewheeling this disconnects gearset 3 (ZF 8HP)
or gearset 1 (GM 8L) from the Powerflow.
So the Powerflow in gears 1 2 3 is in both transmissions also the same.

When you look at the GM 8L90 patent drawing above (and look up the numbers in the patent) you wil notice that they number the gearsets from left to right 3 2 1 4.

If you had numbered them the same way you would have noticed that the equations were exactly the same as for the ZF 8HP :)

Perdo24, great work! So I guess if I had read the patent more closely, and looked at the powerflow like you did, I could have saved the hours it took me to derive all the equations for the 8L :)

Any thoughts on the 10 speed Ford design? The planetary layout is actually rather similar but there is one added shift element (http://www.thetruthaboutcars.com/2014/12/saturation-dive-ford-10-speed-transmission-power-flow/)

Unrelated question – Do you do this sort of thing for a living/education or is this a hobby?

Do you do this sort of thing for a living/education or is this a hobby?
Hobby
It all starts with wanting to know how a planetary gearset works.
And then if you go deeper into it, it becomes more interesting.
The stick diagram design, the hydraulic unit and how it actually shifts gears,
how it is mechanical all connected, the electronics,
from a technician point of view this is all very interesting.

My thoughts about the Ford 10R (and again the great similarity with the ZF 8HP)
I wil post in the comments of your article about the Ford 10R.

Your article about the Ford 10R was very good, using a stick diagram for each gear
and the same color for parts that rotate at the same speed, made it very easy to understood.
You should make a article like this about the Mercedes 9G-Tronic.
I added a link with the powerflow and clutch application of the 9G-Tronic.
It also has an Interactive SVG Nomogram of the 9G-Tronic.

Because I cant post in the comments of your article about the Ford 10R
I wil post it here.

Ford took the ZF 8HP and made a 10-speed out of it.

1) If you remove the clutches C D F (Ford 10R) and the clutches D E (ZF 8HP)
you get the same stick diagram.
2) Ford used the same gearsets 3 and 4 as the ZF 8HP.
Gearset 4 R4:S4 = 85:23 (first gear 4.696 for both transmissions.)
Gearset 3 R3:S3 = 111:69 same as for ZF 8HP.
(R3:S3 = 101:63 is the tooth count you chose)
3) In gears 1 2 3 4 the powerflow and equations for both transmissions are the same.
Also in the highest gear the powerflow and equations are the same
for both transmissions.

If you look at the US Patent #8465390 from 2008 (see article Ford 10R) ZF made a
10-speed out of the ZF 8HP by only adding clutch F, which was a bad design.
I think Ford took a good look at this design and modified it into the 10R.(2012)
The modification was brilliant, by changing the position of the clutches D and E
in the ZF design and adding an intermediate shaft.

In my opinion the stick diagram of the ZF 8HP is the best design there is.
General Motors used it for the 8L90 and now Ford uses it for the 10R.
I also think this will be the only 10-speed automatic (with planetary gearsets)
because no one can make a better.

Perdo24, sometimes the spam filter can be a bit hyperactive. I am just a guest writer here, so there is very little I can do about it.

Excellent points about the Ford 10R and ZF 8HP. When I was doing the research on the Ford 10R, I thought to myself – “Man if ZF had left their engineers some more time to come up with all these variants, they could have made a lot of money with patent license revenue stream”. As US Patent #8465390, clearly there was at least some thought paid to the 4 planetary + 6 shift element design with this type of a gear arrangement. The 9G-Tronic is a completely unique and interesting design. BMW has some patent disclosures with a power flow very similar to the 9G-Tronic. I could write an article about it, but the Daimler paper just covers so much of the details already! I don’t know if I will have too much to add. I am happy to hear that you found the stick diagrams for the Ford 10R design helpful, they certainly took some time to put together. Maybe also because I was trying to learn something more modern than XFig as I was drawing them ;)

No question that the ZF 8HP is a very clever design, but in my opinion the GM 8L90 stick diagram has 3 advantages

1. Lower rotating inertia (as explained in the article) because swapping gear sets 1 and 3 allows for the elimination of outermost tube of the ZF 8HP.
2. Simpler layout of clutch hydraulics ( clutches C, D, and E are connected to a common node, i.e. easier to route hydraulic fluid). Also
3. With a given hydraulic power, shifts will be slightly quicker with the GM 8L90 because of the location of the 3 clutches.

These advantages might be small advantages, but they are refinements none the less. GM has an excellent design patented with 4 gear sets and 6 clutches that does not use/need an intermediate shaft (US Patent 8007394), but not every design can go to production.

Thank you for your in depth thoughts on these transmissions designs, it has certainly given me a different perspective. This is a hobby for me as well, so I can only read about them and try to speculate about the design intent. This is not exactly a very popular hobby either, so it is hard to find people with the interest. Brian P is one of our commentators here and he is very knowledgeable about transmissions and I enjoy his thoughts as well.

Maybe you should consider becoming an author for TTAC yourself!

In case you missed it I also wrote an analysis for the ZF 9HP http://www.thetruthaboutcars.com/2014/02/ur-turn-saturation-dive-into-the-zf-9-speed/